This invention relates to the field of wavelength-tunable semiconductor lasers, and in particular to controllably tuning the lasing wavelength by controlling the optical output power of the laser.
Wavelength-tunable lasers have found important applications in optical communication and sensing. Wavelength-tunable lasers play a central role in particular for dense wavelength division multiplexing (DWDM) systems that form the backbone of today""s optical communication network. The term xe2x80x9cwavelength-tunable laserxe2x80x9d is typically applied to a laser diode whose wavelength can be varied in a controlled manner while operating at a fixed heat sink temperature. At the 1550 nm wavelength regime on which most DWDM systems operate, a wavelength shift of 0.1 nm corresponds to a frequency shift of about 12.6 GHz. At a given heat sink temperature, the central wavelength of a conventional distributed feedback (DFB) laser diode may be red-shifted by as much as 0.3 nm or about 40 GHz due to the rise in the temperature of the junction by Ohmic losses. In contrast, at a given heat sink temperature, the wavelength of a tunable laser may vary by several nanometers, corresponding to hundreds or even thousands of GHz, covering several wavelength channels on the International Telecommunication Union (ITU) grid. Depending on the physical mechanisms of wavelength tuning, the lasing wavelength can be tuned in either positive (red) or negative (blue) direction. Controlled wavelength tunability offers many advantages over conventional fixed wavelength DFB lasers for DWDM operation. It enables advanced all-optical communication networks as opposed to today""s network where optics is mainly used for transmission and the network intelligence is performed in the electronic domain. All-optical networks can eliminate unnecessary E/O and O/E transitions and electronic speed bottlenecks to potentially achieve very significant performance and cost benefits. In addition, a less extensive inventory of wavelength-tunable lasers than of laser with a fixed wavelength is required. Keeping a large inventory of lasers for each and every wavelength channel can become a major cost issue. For advanced DWDM systems, the channel spacing can be as narrow as 50 GHz (or about 0.4 nm in wavelength), with as many as 200 optical channels occupying a wavelength range of about 80 nm. For the reasons stated above, wavelength-tunable lasers have attracted considerable interest in optoelectronic device research.
There exist different design principles for tunable lasers. Almost all wavelength-tunable laser designs make use of either the change of refractive indices of semiconductor or the change of laser cavity length to achieve wavelength tuning. For the former, common mechanisms for index change include thermal tuning, carrier density tuning (a combination of plasma effect, band-filling effect, and bandgap shrinkage effect), electro-optic tuning (linear or quadratic effect), and electrorefractive tuning (Franz-Keldysh or quantum confined Stark effect). For DFB lasers, the wavelength of the laser light propagating in the waveguide is basically determined by the grating period xcex9. The free-space lasing wavelength xcex is given by xcex=2 neff xcex9, where neff is the effective index of refraction of the waveguide and xcex9 is the period for first-order gratings. Accordingly, the change xcex94xcex in the lasing wavelength xcex is directly proportional to the change xcex94n of the index of refraction neff.
Referring to FIG. 1, a prior art three-section DBR tunable laser 100 includes an optical gain section 101, a phase control section 102, and a tunable DBR section 103. A first current source 104 pumps the gain section 102 to generate optical gain; a second current source 105 injects carriers to adjust the phase condition of the phase control section 102 so that the resonant frequency matches approximately the peak of the DBR reflectivity; and a third current source 106 controls the reflectivity peak by changing the effective index neff of the Bragg waveguide section 103. With proper selection of the currents in the DBR region 103 and in the phase control region 102, quasi-continuous wavelength tuning can be achieved. All three sections 101, 102, 103 are optically connected to minimize residue reflections and coupling loss; however, the sections 101, 102, 103 have to be electrically isolated from one another, for example, by layers 107 disposed between the respective sections 101, 102, 103. Three currents, responsible for the gain region, DBR region, and phase control region, have to be supplied; and the lasing wavelength depends on all three currents and is particularly sensitive to the currents in the DBR and phase control region. A continuous wavelength tuning range of about 10 nm can be achieved using this design.
Modifications of the three-section DBR lasers include sampled grated four-section DBR lasers and vernier-tuning sampled grating DBR lasers (not shown). The last device requires four separately controlled current sources to achieve the full tuning range (about 80 nm quasi-continuous tuning).
Alternatively, the lasing wavelength can also be changed by changing the physical length of laser cavity in the surface normal direction. This mechanism has been applied, for example, to vertical-cavity surface-emitting lasers (VCSELs) where typically due to the short cavity length only one or at most very few lasing modes fall within the gain peak. Referring to FIG. 2, a prior art wavelength-tunable VCSEL structure 200 is based on surface micromachining technology. The laser device 200 includes a bottom dielectric DBR mirror 202, a top dielectric DBR mirror 201, an electrostatically controlled membrane 203, and an active region 204. Electrically pumped micro-electro-mechanically tuned VCSEL in the 1550 nm wavelength regime have not yet been demonstrated. However, the laser device 200 can be optically pumped by an incoming pump beam 205 (e.g. a beam from a 980 nm wavelength pump laser) through the bottom mirror 202, with the laser output 206 being emitted from the top mirror 201 disposed on the membrane 203. Wavelength-tuning is obtained by changing the cavity length of the VCSEL through the movement of the membrane 203. With a surface micromachined tunable mirror, a continuous tuning range of 40 nm has been demonstrated with an output power of up to 7 mW coupled to a single mode fiber.
Multiple-section DFB lasers in general have a smaller tuning range than multiple-section DBR lasers, except for the tunable twin-guide (TTG) DFB lasers where relatively wide (about 6 nm) and continuous tuning can be achieved.
In DWDM systems, the wavelength of the channel has to be stabilized within a few gigahertz from the ITU grid, typically less than 10% of the channel spacing. A change of the junction temperature and/or device degradation can cause wavelength drift beyond its acceptable range. Achieving wavelength stability requires monitoring the wavelength in real time using a sophisticated feedback mechanism. Several commercially available devices and their operation for accurately monitoring the laser emission wavelength are shown in FIGS. 3, 4, and 5. Common to these devices is an optical interference device such as a Fabry-Perot etalon placed between the laser and a photodetector. Critical for the device performance are the mechanical stability and angular precision of the etalon and the collimation of the laser beam impinging on the etalon.
Referring now to FIG. 3, a wavelength-monitoring system 300 includes an optical beam splitter 301, a Fabry-Perot (F-P) etalon 302 connected to a first output of the beam splitter 301, a first photodetector (PD) 303 following the F-P etalon 302, with a second PD 304 connected to the second output of the beam splitter 301 as a reference detector. Once the system is calibrated, the lasing wavelength can be determined from the ratio of the photocurrents of the PDs 303, 304, as illustrated in FIG. 4. In the illustrated example showing exemplary target wavelengths xcexn, xcexn+1, a rise in the ratio I1/I2 of the measured photocurren would indicate a decreasing lasing wavelength, while a decrease in the ratio I1/I2 of the measured photocurrents would indicate an increasing lasing wavelength. Note that there exist multiple wavelengths xcexn, xcexn+1, that can yield the correct photocurrent ratio, and each wavelength may correspond to an ITU wavelength channel. This design has problems with generating a proper error signal when wavelength hopping occurs.
FIG. 5 shows a more detailed design of a commercial wavelength monitoring system 400, excluding electronic circuitry. A small fraction, typically a few percent, of the received laser light is coupled into the monitoring system by an optical power splitter 401. The beam is collimated in collimator 402 and then split into two approximately equal signals by a beam splitter 403. The photocurrent of PD 404 provides the reference signal proportional to the power of the received laser light. The photocurrent of PD 406 is related to the received power being transmitted through the Fabry-Perot etalon 405. The ratio of these two photocurrents does not depend on the output power of the received laser light.
The manufacturing and operating complexity of the wavelength-tunable lasers and the wavelength monitoring system represent barriers for the production of low cost wavelength-tunable laser modules for low-cost DWDM systems suitable for metropolitan area networks (MAN). It would therefore be desirable to provide a new design for a wavelength-tunable laser where the lasing wavelength can be tuned by a single current source and the lasing wavelength can be measured without requiring interferometric devices.
According to one aspect of the invention, a wavelength-tunable distributed feedback (DFB) laser structure is disclosed where the lasing wavelength can be adjusted by adjusting a single bias current of the laser diode. Since the output power of the laser diode also increases with the bias current, one can establish a straightforward, one-to-one correspondence between the lasing wavelength and the output power of the laser. Consequently, the lasing wavelength can be measured directly by a power monitoring detector facing, for example, the back-end of the laser diode.
To provide wavelength-tuning, the DFB laser structure includes a second set of quantum wells or a waveguide layer next to the lasing quantum wells as xe2x80x9ccarrier reservoirxe2x80x9d. The second set of quantum wells or the waveguide layer has to meet several requirements in order to function effectively as a carrier reservoir without adversely affecting the laser performance. First of all, the carrier reservoir has to have a higher bandgap than the lasing quantum wells to minimize the optical loss. Secondly, a carrier propagation barrier needs to be present between the lasing quantum wells and the reservoir to avoid carriers falling into the lasing quantum wells, which have the lowest bandgap of all materials and the strongest tendency of attracting carriers. Thirdly, the carrier reservoir has to be located in a region where the intensity of optical field is significant so the carrier induced index change can contribute to the change of lasing wavelength of a DFB laser. Finally, the presence of the carrier reservoir should not trigger high-order transverse modes. In other words, the laser should operate in a single longitudinal mode and single spatial mode. A structure meeting the above requirements includes a reverse-biased tunnel junction made of heavily doped p+- and n30 -layers disposed between two sets of quantum wells to prevent carrier leakage from the reservoir back to the active quantum wells. Because of the carrier tunneling effect, holes can tunnel through the n+/p+ junction and reach the carrier reservoir to meet the electrons. The carrier concentration in the reservoir is then determined by the spontaneous emission rate and Auger recombination rate. This follows approximately the empirical equation I=BN2+CN3 where I is the current, N the carrier concentration in the reservoir, and B and C the rates for spontaneous and Auger recombination, respectively. Contributions from defect-related recombination, which is linearly proportional to the carrier concentration, are neglected.
Embodiments of the invention may include one or more of the following features. The laser can be grown on an n-InP substrate and have a p-InP as the upper cladding layer. A thin layer of material having a higher refractive index than InP can be introduced to form gratings for index-coupled DFB lasers. Two unintentionally doped graded-index (GRIN) confinement regions can be located on either side of the quantum well active layers to provide carrier and optical confinement. Between the two GRIN layers, the quantum wells forming the active layer and responsible for lasing are positioned closer to the p-InP cladding layer and additional quantum wells having a higher ground state energy than the active layer and forming the carrier reservoir are located near the n-InP lower cladding layer.
According to another aspect of the invention, the laser output may be coupled to a semiconductor laser amplifier (SLA) to allow an independent adjustment of the lasing wavelength and the optical output power of the device. Optionally, the laser output may be coupled to an optical modulator, such as an electro-absorption modulator, to externally modulate the laser light to reduce chirping. The modulator may be used with or without the SLA. Detectors may be provided to measure an output power of the laser beam and/or the amplified laser beam and/or the modulated laser beam.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.